This application is based on applications Nos. H11-044481 and H11-044488 filed in Japan on Feb. 23, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image display apparatus that is, when in use, placed in front of an eye of an observer, and particularly to an image display apparatus that employs a reflection-type liquid crystal display panel to display images.
2. Description of the Prior Art
There is a type of image display apparatus that is, when in use, kept in front of an eye of an observer by being mounted on the head or held with the hand of the observer. Such image display apparatuses are widely used as a means for presenting virtual reality with much realism or as a viewfinder incorporated in an image shooting apparatus such as a video camera. This type of image display apparatus is so constructed that the light of a displayed image is directed through an observation optical system to the eye of the observer so as to present an enlarged virtual image of the displayed image to the observer.
Here, it is desirable that the image be presented to the observer with high brightness, high resolution, and, in particular where virtual reality is presented, a wide angle of view. On the other hand, considering that the apparatus needs to be mounted on the head or held with the hand of the observer, it is highly desirable that the apparatus be made compact and light. These requirements are met by using a liquid crystal display panel, and this is the reason that most image display apparatuses of the type used in front of an observer""s eye adopt a liquid crystal display panel.
A liquid crystal display panel has a large number of pixels arranged in a two-dimensional array, and modulates the illumination light shone thereon by performing polarization conversion on the illumination light on a pixel by pixel basis so as to obtain desired intensity distribution of differently polarized light components in the thus converted illumination light. The polarization conversion is controlled pixel by pixel in accordance with an image signal so that different degrees of polarization conversion will be performed for the individual pixels. This produces differences in the amounts of differently polarized light components among the pixels, and directing such light including varying amounts of differently polarized light components to the eye enables the eye to see an image in the form of a pattern of varying brightness.
Liquid crystal display panels are roughly grouped into transmission-type and reflection-type liquid crystal display panels, of which the former receive illumination light from the side opposite to the side from which the image is observed and the latter receive illumination light from the same side from which the image is observed. Reflection-type liquid crystal display panels offer the following advantages as compared with transmission-type liquid crystal display panels. In a liquid crystal display panel, the individual pixels are controlled by circuits such as TFTs (thin-film transistors), which themselves occupy a certain area. Whereas these circuits cannot be arranged without reducing the apertures of the individual pixels in transmission-type liquid crystal display panels, they can be arranged on the face opposite to the side from which the image is observed in reflection-type liquid crystal display panels, which therefore suffer less from loss in aperture ratio due to the arrangement of such circuits and thus offer brighter images.
The difference in aperture ratio between transmission-type and reflection-type liquid crystal display panels becomes more striking as the pixels are made smaller. Accordingly, if the same brightness is aimed at with the same number of pixels, reflection-type liquid crystal display panels can be made more compact. Conversely, if the same size is aimed at, reflection-type liquid crystal display panels can be provided with more pixels, making it possible to present images of higher resolution.
Moreover, reflection-type liquid crystal display panels by nature allow the liquid crystal layer by which they perform polarization conversion to be thinner than in transmission-type liquid crystal display panels. This permits reflection-type liquid crystal display panels to refresh the displayed image faster.
Thus, using a reflection-type liquid crystal display panel having these advantages contributes greatly to obtaining bright and high-resolution images as desired in an image display apparatus used in front of an observer""s eye. In addition, the smaller size of the display panel allows the use of a smaller observation optical system and thereby facilitates the miniaturization of the apparatus as a whole.
The observation optical system serves to direct light from the display panel to the observer""s eye, and therefore it is desirable that the observation optical system be not only capable of presenting the displayed image to the observer without loss in image quality but also sufficiently compact and light to suit the intended uses. From this perspective, it has been proposed to provide the observation optical system with a reflecting surface having an optical power.
Whereas a refracting surface is associated with the dependency of the angle of refraction on the wavelength, a reflecting surface is free from such wavelength-dependence of the angle of reflection, and thus does not cause chromatic aberration. Moreover, a reflecting surface, despite having a positive power that makes light converge, offers a negative Petzval value, and thus contributes to improving the Petzval sum of the entire optical system, making it possible to almost eliminate image-surface distortion even at the edge so as to present images with excellent flatness. In addition, a reflecting surface allows the optical paths of incident and reflected light to overlap partially, and thus makes it easier to secure a relatively long total optical path. Accordingly, an observation optical system provided with a reflecting surface having an optical power is compact but nevertheless offers a higher magnification and a wider angle of view without any loss in image quality.
Head-mounted display (HMD) apparatuses having an observation optical system provided with a reflecting surface having such advantages are disclosed in U.S. Pat. Nos. 5,777,794, No. 5,701,202, and No. 5,659,430.
As described above, a reflection-type liquid crystal display panel needs to receive illumination light from the side from which the image is observed, and therefore the optical paths of illumination and reflected light overlap. Accordingly, it is necessary to separate the unmodulated illumination light coming from the light source and the modulated reflected light coming from the liquid crystal display panel. However, the HMD apparatuses disclosed in the above-mentioned patents are not so constructed as to allow separation of the illumination and modulated light, and therefore it is impossible to use a reflection-type liquid crystal display panel as a display device therein. For this reason, these HMD apparatuses employ a transmission-type liquid crystal display panel despite its somewhat inferior performance, and therefore, with them, it is impossible to achieve a satisfactory improvement in the quality of the images presented to the observer even though they have an observation optical system provided with a reflecting surface.
On the other hand, an HMD apparatus employing a reflection-type liquid crystal display panel has also been proposed, of which the construction is shown in FIG. 7. In this HMD apparatus, between a reflection-type liquid crystal display panel 101 and a light source 102 for illuminating it, a PBS (polarized-beam separating) mirror 103 is disposed that transmits one and reflects the other of two polarized light components having mutually perpendicular polarization planes. In addition, in the optical path of the light reflected from the liquid crystal display panel 101 and then reflected from the PBS mirror 103, an eyepiece lens 104 is disposed as an observation optical system. The reflected light coming from the liquid crystal display panel 101 is separated from the illumination light coming from the light source 102 by the PBS mirror 103, and is then directed through the eyepiece lens 104 to the observer""s eye EP.
In this HMD apparatus, the use of the reflection-type liquid crystal display panel 101 helps display high-quality images. However, since the observation optical system here is composed solely of refracting surfaces, increasing the magnification tends to make the quality of the images presented to the observer poorer, making it difficult to obtain a satisfactorily wide angle of view. Moreover, even though the eyepiece lens 104 serving as the observation optical system is designed to make the best of the reflection-type liquid crystal display panel 101 that excels in high definition, this can be achieved only within a certain limit, and, in particular where there are restrictions on the size, it is simply difficult to design the eyepiece lens to make the best of the reflection-type liquid crystal display panel.
Moreover, in this HMD apparatus, the PBS mirror 103 is used to separate the illumination light and the light reflected from the reflection-type liquid crystal display panel 101 and also to separate the image-conveying and no-image-conveying polarized light components included in the reflected light, but the polarization separation achieved by the PBS mirror 103 is not always perfect. In general, a PBS mirror exhibits varying polarization separation characteristics according to the angle of incidence of the light striking it; specifically, if light strikes it at an angle of incidence outside the designed range, the polarized light component that should be transmitted is partially reflected and the polarized light component that should be reflected is partially transmitted. Moreover, although the illumination light as a whole is directed to the reflection-type liquid crystal display panel from a direction perpendicular thereto, it includes rays that travel obliquely relative to the reflection-type liquid crystal display panel and that thus strike the PBS mirror at angles of incidence outside the designed range. The imperfectness of the polarization separation achieved by the PBS mirror appears in both the separation of the illumination and reflected light and the separation of the image-conveying and other polarized light components included in the reflected light.
As a result of imperfect polarization separation, some light that conveys no image is directed to the observer""s eye. This light causes, for example, a ghost, and thereby degrades the quality of the images presented. This greatly devalues the significance of using a reflection-type liquid crystal display panel that itself offers excellent performance.
An object of the present invention is to provide an image display apparatus that presents images with higher quality than ever despite being sufficiently compact and light to suit the intended uses.
To achieve the above object, according to one aspect of the present invention, an image display apparatus used in front of an observer""s eye is provided with: a reflection-type modulation device for modulating illumination light incident thereon in accordance with an image displayed thereon and reflecting back the thus modulated illumination light as image light; a semi-transmissive reflection device for directing the illumination light to the reflection-type modulation device and letting the image light reflected back from the reflection-type modulation device emerge in a direction that is not parallel to the direction from which the semi-transmissive reflection device receives the illumination light; an observation optical system for directing the image light having emerged from the semi-transmissive reflection device to the observer""s eye so that a virtual image of the image displayed on the reflection-type modulation device will be presented to the observer; and a separation device disposed in the optical path of the image light between the reflection-type modulation device and the observer so as to separate the light other than the image light from the light directed to the observer""s eye.
According to another aspect of the present invention, an image display apparatus used in front of an observer""s eye is provided with: a reflection-type modulation device for modulating illumination light incident thereon in accordance with an image displayed thereon and reflecting back the thus modulated illumination light as image light; a semi-transmissive reflection device for directing the illumination light to the reflection-type modulation device and letting the image light reflected back from the reflection-type modulation device emerge in a direction that is not parallel to the direction from which the semi-transmissive reflection device receives the illumination light; and an observation optical system for directing the image light having emerged from the semi-transmissive reflection device to the observer""s eye so that a virtual image of the image displayed on the reflection-type modulation device will be presented to the observer. Here, the observation optical system includes a reflecting surface having an optical power.